Conventional fuel systems for vehicles with internal combustion engines can include a fuel vapor canister that accumulates fuel vapor from a headspace of a fuel tank. If there is a leak in the fuel tank, the canister, or in any associated pipes, conduits, hoses and connections within the system, fuel vapor could escape through the leak and be released into the atmosphere instead of being accumulated in the canister. Various government regulatory agencies, including the U.S. Environmental Protection Agency (USEPA) and the California Air Resources Board (CARB, a part of the California Environmental Protection Agency (Cal/EPA)), have promulgated standards related to limiting fuel vapor releases into the atmosphere. There is therefore a need to avoid releasing fuel vapors into the atmosphere, and to provide an apparatus and a method for performing a leak diagnostic, so as to comply with those standards.
Atmospheric pressure is generally defined as the downward pressure exerted by the weight of the overlying atmosphere. It is generally referred to as one atmosphere of pressure at sea level, with some variation due to altitude and weather conditions. Pressure can be described in absolute terms (e.g. 14.7 lbs/square inch) or in differential (or relative) terms (e.g. lower than atmospheric pressure inside a fluorescent lamp or higher than atmospheric pressure inside a SCUBA tank). In the present application, the pressure differential between the sealed environment within an automotive fuel vapor containment system and a reference pressure such as the ambient atmospheric pressure is of interest. Excessive pressure differentials signifying excessively low pressure or excessively high pressure within an automotive fuel vapor containment system could cause the integrity of the fuel system to be compromised. In this regard, the ability to prevent excess pressure differential is desirable for safety, system performance and longevity.
One method in use for detecting fuel system leaks is known as natural vacuum leak detection (NVLD). The method was presented at the SAE-Toptech Conference, Indianapolis, 1999. In that method, the fuel system, including the fuel tank, fuel vapor canister, and associated pipes, conduits, hoses and connections within the system, are sealed from the atmosphere immediately after an engine shut-down for a pre-determined time. During most situations in which a sealed automotive fuel system is not operating, a vacuum develops (pressure below atmospheric pressure) inside the fuel system due to gas law effects, principally from cooling of the tank. A vacuum switch changes state at a specified vacuum level, and that change in state is detected by a processor. If a sufficient vacuum (a sufficiently low pressure) is reached in the system to trip or maintain the switch in the vacuum state over a specified period of time, then the system is deemed to pass the leak test. If sufficient vacuum to change the state of the vacuum switch is not reached or if the vacuum “decreases” too rapidly (i.e. approaches atmospheric pressure too rapidly), the system does not pass the leak test.
FIG. 1 is a schematic illustration of a prior art fuel system capable of NVLD. Referring to FIG. 1, a fuel system 100 for an engine (not shown), includes a fuel tank 110, a vacuum source 130 such as an intake manifold of the engine, a purge valve 140, a fuel vapor canister 150, and an electronic control unit (ECU) or processor 160 with memory storage 170.
Additionally, the fuel system 100 may comprise several components for managing fuel vapor pressure. A vacuum switch 184 signals the ECU 160 that a first predetermined pressure (vacuum) level exists. The vacuum switch 184 may be activated by movement of a diaphragm in response to a pressure differential across the diaphragm.
The fuel system 100 may further comprise a “vacuum relief” device 186 for relieving excessive negative pressure at a value below the first predetermined pressure level, and a “pressure relief” device 188 for relieving excessive positive pressure above a second predetermined pressure level. Both the vacuum relief and pressure relief devices use port 190 which is vented to the atmosphere (i.e. atmospheric pressure) for either drawing air into or venting vapor from the system.
Other functions are also possible. For example, in connection with the operation of the purge valve 140 and a logic process performed by the ECU 160, the system 100 can perform large leak detection during operation. Such large leak detection is used to evaluate situations such as when a refueling cap 120 is either not replaced on the fuel tank, installed incorrectly or otherwise not properly sealing as it was designed to do.
Volatile liquid fuels, including gasoline, can evaporate under certain conditions, such as rising ambient temperature, thereby generating fuel vapor. In the course of cooling that is typically experienced by the fuel system 100 after the engine is turned off, a vacuum is naturally created by cooling of the fuel vapor and air, such as in the fuel tank headspace 115 of the fuel tank 110 and in the canister 150. In accordance with the NVLD test described above, the existence of an acceptable level of vacuum at the first predetermined pressure level at the vacuum switch 184 thru the duration of the test indicates that the integrity of the fuel system 100 is satisfactory, so that system passes the test. Conversely, if the vacuum switch 184 signals the ECU 160 indicating the predetermined level of vacuum is not reached or decreases too rapidly (i.e. approaches atmospheric pressure too rapidly), that indicates the integrity of the fuel system 100 is unsatisfactory, and that the system does not pass the test. As noted below in the discussion regarding type I and type II errors, failing or passing the test does not necessarily mean the fuel system 100 does or does not have an actual leak.
If pressure in the fuel system 100 falls below the first predetermined pressure level, indicating that excessive vacuum is present, the vacuum relief device 186 protects the fuel system 100 from damage by allowing outside air to enter the system through port 190.
Additionally, if pressure in the fuel system 100 rises above a second predetermined level, indicating a pressure well above what the system would normally see, the pressure relief device 188 allows air within the fuel system 100 to be released through port 190 while fuel vapor is retained. Over-pressure of the system is not desired as it could also compromise the integrity of the fuel system 100 by causing possible structural, component or interconnection failure. In the course of refueling the fuel tank 110 through filler cap 120, the pressure relief device 188 may allow air to exit the fuel tank headspace 115 at a high rate of flow. In addition, during a high rate purge of the fuel tank 120, the purge valve 140 may assist in dissipating the high pressure (through a port not shown). That function is commonly referred to as Onboard Refueling Vapor Recovery (ORVR).
As an electromechanical device, the vacuum switch 184 has an activation threshold and a tolerance (error) around the activation required to make a state change. That tolerance must be taken into consideration in order to avoid excessive type I (alpha or false positive) and type II (beta or false negative) errors. An example of a type I error in this context is a leak test result indicating the system has a leak when in reality it does not. An example of a type II error in this context is a leak test result indicating the system has no leak when in reality it does. Systems, particularly sensors and processors, must be statistically capable of discriminating signals from noise and yield acceptable levels of system errors. For this example, a type I error would be an annoyance for a consumer or could cause a fleet owner to lose revenue while a vehicle is out of service un-necessarily. A type II error could cause a vehicle to be unnecessarily venting fuel or fuel vapors to the atmosphere.
On board diagnostics II (OBD II) for detection of evaporative system (EVAP) leaks have been required since the automotive model year 1996 by both the USEPA and Cal/EPA. The OBD II system subjects the fuel tank, vapor lines and fuel vapor canister, and all other components of the fuel system to either vacuum or pressure (manufacturer choice). If the system detects no airflow when the EVAP canister purge valve is opened, or it detects a leakage rate that is greater than that which would pass through a hole 0.040″ (1 mm) in diameter (0.020″ or 0.5 mm for 2000 and up model year vehicles), it indicates a fault code to the computer.
The vacuum relief device 186 and pressure relief device 188 of many current fuel systems are solenoid actuated valves. A solenoid valve is an electro-mechanical valve actuated by changing an electrical current through a solenoid, thus changing the state of the valve (i.e. from open to closed or from closed to open), depending on the valve design and the “normal” position. A valve normal position is defined as the state in which it will rest when current is removed. Solenoid valves may be normally open or normally closed. To control flow through a solenoid valve, a method called pulse width modulation (PWM) is used. PWM involves modulating the duty cycle and the modulation rate. Because solenoid valves are electro-mechanical devices and have high duty cycles and modulation rates, they have frequent reliability issues and consume processing bandwidth of an ECU.
It would therefore be desirable to improve the reliability of the leak detection, pressure relief and vacuum relief systems of the fuel system described above. It would furthermore be desirable to provide a simple system for performing leak detection and pressure relief having a high level of performance and accuracy, in a cost-effective, compact configuration.